Frequently in drilling an oil or gas well the well borehole penetrates earth formations which may collapse around the drill string and plug the annular space between the drill string and the wall of the borehole. This can cause the drill string to become stuck due to differential pressure between the pressure in the borehole and that in the formation outside the borehole. Similarly drill string or tubing may become stuck due to pressure differentials between the borehole and the formation surrounding the borehole, if over a long interval, the pipe comes in contact with one wall of the borehole such as can be caused by the axis of the borehole drifting from its original vertical line. In either of these events pressure differential between the borehole and the earth formations can cause the drill string or a tubing string suspended in the borehole to become stuck against the earth formation having a lower pressure than the pressure of the drilling fluid in the well borehole. This is always a potential problem in open hole operations.
When this problem occurs remedial operations are required. A typical remedial operation is to unthread or cut by a shaped charge or chemical cutter the pipe or tubing string at a point just above the location where the differential pressure causes the sticking. In other words, the upper portion of the stuck string of pipe or tubing are severed from the lower portion and removed from the borehole. Subsequent alternate operations such as drilling can then be undertaken to remove the lower portion of the stuck pipe string. In these situations it becomes very important to determine the depth level at which the pipe or drill string is stuck in the borehole. Techniques for doing this are known as freepoint indicating techniques.
The art is replete with methods and apparatus for determining freepoint of stuck pipe. One recently developed technique which is assigned to the assignee of the present invention is that shown in U.S. Pat. No. 4,440,019 to Marshall. In the technique of the Marshall Patent a freepoint indicating tool having a magnetic field coil is lowered into a stuck string of pipe or tubing. As the freepoint indicator is moved along the stuck pipe string or tubing string the magnetic coil in the pipe is pulsed with direct current causing an intense magnetic field to be generated in the vicinity of the non-magnetic body of the freepoint indicator. This magnetic field magnetizes or causes magnetic marks on the walls of the tubing string or pipe string as the freepoint indicator is moved through the string of pipe or tubing. Residual magnetism from these marks is then detected by lowering the tool again to a location past where the marks were begun and by moving the coil up the tubing string and this time using it as a sensing coil to sense the magnetic fields caused by the residual magnetism left from the magnetic marks. If a torque or tension is applied to the string of stuck pipe or tubing the portion of the string of stuck pipe or tubing above the stuck point or freepoint can strain or deform due to the torque or tension applied thereto. This strain causes a changing of the residual magnetic field in the pipe at those locations where the magnetic marks have been made. A second pass using the coil as a detector coil is then made past the magnetic marks and the amplitude of each mark is examined and compared with the amplitude of the mark as recorded prior to the tensioning or torquing of the pipe or tubing string. Thus by comparing the magnetic marks before and after the torquing or tensioning operation, changes in amplitude can be used to indicate the point at which the pipe is stuck in the well borehole.
One problem which is encountered in this type of freepoint indicating system is that when using the coil as a detector rather than a marker, the voltage which is induced in the coil by moving it past the residual magnetic marks on the pipe is proportional to the number of turns in the coil (which of course remains constant) but is also proportional to the speed of movement of the instrument past the magnetic mark. As it is normally desired to move the instrument at a constant speed past the magnetic marks this normally does not present a problem. However if the pipe has scale, rust or other irregularities on the inside surface thereof the tool may move in a jerky fashion and may be accelerated starting and stopping due to tensioning in the cable and grappling of the tool with the inside surfaces of the pipe or tubing string causing resistance to its movement in a uniform manner. Thus the assumption that the tool is moving at a constant speed past the magnetic marks when using the coil as a detector may not always be valid. This can lead to false indications of change in the residual magnetic field intensity caused by tool movement rather than by actual changes in the magnetic field due to tensioning or torquing the pipe. This problem can be overcome by using a freepoint indicating system in accordance with the concepts of the present invention.
In the present invention the magnetic field sensing means which are used are sensitive to the magnetic field independent of the speed of movement of the instrument past the magnetic marks placed on the interior of the stuck pipe or tubing string.